This application is based on Japanese Patent Application Nos. 11-7724 (1999) filed Jan. 14, 1999, 11-141955 (1999) filed May 21, 1999, and 11-361728 filed Dec. 20, 1999, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a light generation method and a light source for generating a single-mode incoherent light having a low intensity noise and a small spectral bandwidth, and more particularly, to a light generation method and a light source for using a wavelength-tunable optical filter to output a single-mode light having wavelength components in a particular band of a white-light band by obtaining this single-mode light from a white-light having wavelength components over a wide-band in a wavelength domain.
2. Description of the Related Art
Single-mode light sources are configured to obtain a single-mode light by using an optical filter to spectrum-slice a white-light having an emission spectrum spreading over a wide-band in a wavelength domain. The single-mode light refers to a light showing a uni-modal spectrum distribution around a particular wavelength.
In addition, the white-light refers to a light having continuous spectral components over a wide-band in a wavelength domain and is also referred to as a Gauss light.
A conventional single-mode light source of this kind is typically comprised of a white-light source 81 and an optical filter 90 as shown in FIG. 21, and also has an isolator 82 located in an output section of the white-light source 81 for preventing an unwanted light returning from the optical filter 90. That is, such a light source is comprised of the wide-band white-light source 81 for generating a wide-band white-light, the wavelength-tunable optical filter 90 having a particular transmission band, and the isolator 82 for preventing an unwanted light returning from the wavelength-tunable optical filter 90 so that a white-light from the wide-band white-light source 81 is filtered when it passes through the wavelength-tunable optical filter 90 via the isolator 82.
The white-light source 81 may be comprised of an incandescent lamp, a super luminescent diode (SLD), or an amplified spontaneous emission (ASE) generated from an optical amplifier. The optical filter 90 may be comprised of a dielectric multilayer film filter, an acoustooptical filter, or a grating monochromator.
A white-light from the white-light source 81 has wavelength components over a wide-band in a wavelength domain. The single-mode light source for obtaining a single-mode light by spectrum-slicing a white-light using the wavelength-tunable optical filter is a mode-hop-free light source that replaces a wavelength-tunable single-mode laser light source, and is conventionally used not only for optical measurements but also as a simple light source for telecommunications systems based on wavelength-division multiplexing (WDH). The spectrum slicing refers to transmitting a white-light through the wavelength-tunable optical filter to obtain a single-mode light having wavelength components in a particular narrow band of the white-light band.
FIG. 22 shows a mechanism for obtaining a single-mode light by using a filter to spectrum-slice an arbitrary center transmission wavelength of a wide-band white-light. As shown in this figure, the spectral shape of a sliced single-mode light reflects a transmission wavelength characteristic of the filter, but the use of an optical filter having a tunable transmission wavelength enables the center transmission wavelength to be controlled using only the optical filter.
In addition, some single-mode light sources are comprised of a combination of a white-light source and an optical filter to spectrum-slice a single-mode light of a selected wavelength from a wide-band white-light. The wide-band white-light source may be comprised, for example, of an amplified spontaneous emission (ASE) generated from an optical fiber amplifier typically including an erbium-doped fiber amplifier (EDFA). Since a spectrum of an ASE from an optical fiber amplifier generally has no fine structure, a single-mode light can be obtained which has an arbitrary center transmission wavelength xcexc selected by the optical filter. In addition, an arrayed waveguide grating (AWG) filter can be used to simultaneously obtain single-mode lights of a plurality of wavelengths.
The conventional single-mode light sources, however, have the following problems: since the optical filter filters a white-light occurring in a wide wavelength domain, the output of the resulting single-mode light is very small. Furthermore, the minimum value of the wavelength spectral bandwidth of the single-mode light obtained and the extinction ratio of lights generated in the overall wavelength spectrum except for its portion corresponding to a center transmission wavelength are limited by the performance of the optical filter used. In addition, since an emission phenomenon in the wavelength domain of a light transmitted through the optical filter is a probabilistic event in terms of the emission in the overall wavelength spectrum, the single-mode light obtained has intensity noise that is likely to increase with decreasing transmission wavelength spectral bandwidth of the optical filter.
That is, the wide-band white-light source 81 of the conventional single-mode light source is comprised of a SLD or an ermium-doped optical fiber amplifier (EDFA) which provides high outputs. If, however, a white-light from such a light source is spectrum-sliced, the output of the resulting single-mode light is very small. If, for example, a white-light uniformly output at 10 mW over a 100-nm band is spectrum-sliced at a bandwidth of 0.1 nm, the output of the resulting single-mode light is 10 xcexcW at most.
Thus, an attempt is made to amplify such a faint single-mode light using an optical amplifier, but simple amplification does not induce a sufficient emission and a spontaneous emission amplified by the optical amplifier occurs in a band around the single-mode light, thereby significantly degrading the spectral purity of the single-mode light. Such degradation causes the signal-to-noise ratio in both optical communication and measurements systems.
For the optical communications systems based on the WDM technique of multiplexing signals into different wavelengths in the wavelength domain, a light source has been desired to have a low intensity noise and a high spectral purity sufficient to restrain wavelength components other than those of the signal light, in order to prevent the signal-to-noise ratio from being degraded
In addition, the conventional single-mode light source for spectrum-slicing a white-light slices a narrow-band single-mode light from a wide-band light source, so that it has an inherent intensity noise within a short observation period as shown below.
If arbitrary beams are observed over a definite period of time (T), the probability PT(m) of finding (m) photons in this period is expressed by the following equation:
P(m)=∫0∞p(m,xcexd)W(xcexd)dxcexdxe2x80x83xe2x80x83(1)
where p(m, xcexd) denotes a probability density function for the probability of finding (m) photons in an independent population having an average photon flow rate (xcexd) and W(xcexd) denotes a probability density function for the average photon flow rate (xcexd). The population means photons that belong to an identical emission phenomenon in a ring. Counting statistics for such a population conforms to the Poisson distribution, so that the following equation is established.
p(m,xcexd)=(xcexdm/m!)exp(xe2x88x92xcexd)xe2x80x83xe2x80x83(2)
A chaotic light source such as a wide-band light source is a class of such identical populations each of which meets the poisson distribution in equation (2). However, in photon counting statistics limiting the wavelength band, the probability density function W(xcexd) for the average photo flow rate (xcexd) of all populations attenuates as shown by the following expression:
W(xcexd)=(1/xcexc)exp(xe2x88x92xcexd/xcexc)xe2x80x83xe2x80x83(3)
where (xcexc) designates the average of the average photon flow rates of different populations. Thus, the photon counting statistics in short observation period for beams obtained by spectrum-slicing the white-light is expressed as follows:                                                                         P                ⁡                                  (                  m                  )                                            =                                                ∫                  0                  ∞                                ⁢                                  xe2x80x83                                ⁢                                                                            ν                      m                                                              m                      !                                                        ⁢                                      xe2x80x83                                    ⁢                                      exp                    ⁡                                          (                                              -                        ν                                            )                                                        ⁢                                      1                    μ                                    ⁢                                      xe2x80x83                                    ⁢                                      exp                    ⁡                                          (                                              -                                                  xe2x80x83                                                ⁢                                                  ν                          μ                                                                    )                                                        ⁢                                      xe2x80x83                                    ⁢                                      ⅆ                    ν                                                                                                                          =                                                μ                  m                                                                      (                                          1                      +                      μ                                        )                                                        1                    +                    m                                                                                                          (        4        )            
On the other hand, in a long observation period, counting photons for all spectra results in a fixed average at any point of time because all populations are subjected to counting. Consequently, the probability density function is a delta functionxcex4 (xcexdxe2x88x92xcexc) even for the chaotic light and conforms to the poisson distribution.
The photon counting statistics shown by Equation (4) indicates that the photon flow rate substantially fluctuates among the short observation periods, that is, indicates the presence of intensity noise. Thus, since the current optical communication systems using the method for directly modulating and detecting optical signals identify data based on the amount of photons counted in terms of time slots corresponding to bits, it cannot accommodate a large intensity noise such as one shown by Equation (4).
The present invention has been provided in view of these problems, and it is an object thereof to provide a light generation method and a light source that are preferable in obtaining a single-mode light having a high output, a small wavelength spectral bandwidth, and a low intensity noise.
It is another object of the present invention to provide a stabilized single-mode light source that can generate an incoherent single-mode light at an arbitrary wavelength which has a small spectral bandwidth and a restrained intensity noise.
It is yet another object of the present invention to provide a light generation method and a light source that are preferable in obtaining a high-output single-mode light without degrading the spectral purity of the single-mode light.
To attain these objects, the present invention carries out, at least once, the process of using an optical amplifier to amplify a single-mode light obtained by filtering a white-light by means of an optical filter, and then filtering an amplified light using an optical filter that has a center transmission wavelength equal to that of the above optical filter, so that the light intensity is increased by passage through the optical amplifier a large number of times, while the wavelength spectral bandwidth is reduced by passage through the optical filter a large number of times for filtering.
With this configuration, if a transmission wavelength characteristic of an optical filter is defined by T(xcex), a wavelength spectrum xcfx81(xcex) of a single-mode light passing through a large number of optical filters having an equal center transmission wavelength is expressed as follows:
xcfx81(xcex)=T(xcex)xc2x7T(xcex)xc2x7 . . . xc2x7T(xcex)xc2x7T(xcex) . . . xe2x80x83xe2x80x83(5)
Thus, a single-mode light can be obtained that has a much smaller wavelength spectral bandwidth than a light obtained after a single passage through the optical filter.
Furthermore, a light generation method according to the present invention uses a simple configuration consisting of a set of optical amplifiers and filters to increase outputs while reducing the wavelength spectral bandwidth by allowing a single-mode light obtained by filtering a white-light to propagate through a path having the optical amplifiers and filters alternatively connected together. The optical amplifier also works as a white-light source covering a wide-band.
The optical amplifier can be used as a wide-band white-light source because an optical gain medium of the optical amplifier enters an inverse distribution state to obtain a gain required for optical amplification, whereby a spontaneous emission, which is low when excited, is amplified during propagation through the optical amplifier before output. Such a light is referred to as an xe2x80x9camplified spontaneous emission (ASE)xe2x80x9d and characterized by its wide-band unique to the optical amplifier and its outputs higher than those of light emitting diodes.
To implement this method, the present invention constructs an optical ring by allowing an output from the optical amplifier to enter the optical filter, where it is filtered and transmitted, and by branching a light obtained and finally feeding one of the split lights back to the optical amplifier. An isolator or the like is inserted into the optical ring constructed, so that the effect set forth in Claim 1 can be obtained because the light undergoes the effects of optical amplification and filtering a large number of times while circulating through the optical ring in one direction. Since, however, a light output is obtained from a branching device provided in the optical ring, a wavelength spectrum xcfx81(xcex) of a single-mode light obtained shows a reduced width compared to Equation (5). That is, if the transmission wavelength characteristic T(xcex) of the optical filter is used and an intensity change rate per circulation through the optical ring is defined as (xcex3):                               ρ          ⁡                      (            λ            )                          =                                            T              ⁡                              (                λ                )                                      +                          γ              ⁢                              xe2x80x83                            ⁢                                                T                  ⁡                                      (                    λ                    )                                                  2                                      +                                          γ                2                            ⁢                                                T                  ⁡                                      (                    λ                    )                                                  3                                      +            …                    =                                    T              ⁡                              (                λ                )                                                    1              -                              γ                ⁢                                  xe2x80x83                                ⁢                                  T                  ⁡                                      (                    λ                    )                                                                                                          (        6        )            
then the wavelength spectral bandwidth is substantially affected by the intensity change rate (xcex3). In general, when the intensity change rate is close to 1 where divergence occurs, Equation 6 provides, at the center transmission wavelength (the wavelength at T=1), a wavelength spectral bandwidth gradually approaching zero. Specifically, if T(xcex) is a Lorentzian transmission function and a full width at half maximum (FWHM) is 0.1 nm, the line width of an output light is 0.01 nm at xcex3=xe2x88x920.05 dB. This is a sufficient reduction in wavelength spectral bandwidth because typical optical filters such as grating filters or dielectric multilayer film filters have an FWHM of 0.1 nm or less.
When the gain per circulation becomes excessive, the optical ring exceeds its oscillation threshold to start laser oscillation due to its configuration similar to that of a ring laser oscillator. Such laser oscillation, however, has a problem that it is so sensitive to fluctuations in optical-ring length at a wavelength level as to generate a large intensity noise when the oscillation state rapidly changes to a non-oscillation state. Thus, the present invention controls the circulation gain of the optical ring to prevent such laser oscillation. Specific means for controlling the circulation gain include, for example, means based on gain control of optical amplification used for the optical ring and means based on adjustments of attenuation provided by a variable optical attenuator inserted into the optical ring.
Furthermore, the present invention employs an optical amplifier having gain saturation to achieve a reduction in light intensity noise, which is the second object of the prior art.
Intensity noise in a single-mode light obtained by filtering a white-light using an optical filter is essentially a quantum optical element associated with an emission process. That is, the light intensity is equivalent to the number of photons counted per unit time, and a probability PT(m) of detecting (m) photons if a light is observed over a finite period of time (T) can be written as follows:
PT(m)=∫0∞p(m,xcexd)W(xcexd)dxcexdxe2x80x83xe2x80x83(7)
where p(m,xcexd) denotes a probability of detecting (m) photons in an independent population having an average photon flow rate (xcexd) and W(xcexd) denotes a probability distribution function for the average photon flow rate (xcexd) of all populations. The population refers to a minimum unit for an independent group of emission events that are correlated to one another. In such a population, photon counting statistics for the probability p(m,xcexd) of detecting (m) photons follows the poisson distribution, so that the following equation is established:                               P          ⁡                      (                          m              ,              ν                        )                          =                                            ν              m                                      m              !                                ⁢                      xe2x80x83                    ⁢                      exp            ⁡                          (                              -                ν                            )                                                          (        8        )            
For a single-mode light obtained by filtering a white-light using an optical filter, events randomly occur in which a light is emitted within the transmission wavelength band of an optical filter, whereby an average distribution of chaotic populations is given for the chaotic light as follows:                               W          ⁡                      (            ν            )                          =                              1            μ                    ⁢                      xe2x80x83                    ⁢                      exp            ⁡                          (                              -                                  xe2x80x83                                ⁢                                  ν                  μ                                            )                                                          (        9        )            
where xcexc is the average of the average photon flow rates of the different populations. Thus, the probability PT(m) of detecting (m) photons during observations over the definite period of time (T) is expressed as follows:                                                                                           P                  T                                ⁡                                  (                  m                  )                                            =                                                ∫                  0                  x                                ⁢                                  xe2x80x83                                ⁢                                                                            ν                      m                                                              m                      !                                                        ⁢                                      xe2x80x83                                    ⁢                                      exp                    ⁡                                          (                                              -                        ν                                            )                                                        ⁢                                      1                    μ                                    ⁢                                      xe2x80x83                                    ⁢                                      exp                    ⁡                                          [                                              -                                                  xe2x80x83                                                ⁢                                                  ν                          μ                                                                    ]                                                        ⁢                                      xe2x80x83                                    ⁢                                      ⅆ                    ν                                                                                                                          =                                                μ                  m                                                                      (                                          1                      +                      μ                                        )                                                        1                    +                    m                                                                                                          (        10        )            
Consequently, the probability PT(m) behaves similarly to a chaos light. The chaotic light refers to a light such as a blackbody radiation. Although the results of measurements based on temporal averaging for the wavelength domain indicate that such a single-mode light shows a stable intensity distribution based on the ergodic theorem, the light shows intensity noise in a time domain. The ergodic theorem refers to a case in which the average of the populations equals the temporal average.
Such fluctuations in the number of photons (that is, intensity noise) can be reduced using the gain saturation of optical amplifiers. The previous paper xe2x80x9cAmplitude squeezing in a semiconductor laser using quantum nondemolition measurement and negative feedback,xe2x80x9d Y. Yamamoto, N. Imoto, and S. Machida, Phys. Rev. A, Vol.33(5), pp. 3243-3261 (1986) clarifies that fluctuations in the number of photons in a single-mode laser light similar to a coherent light can be reduced using the gain saturation of optical amplifies. This technique, however, squeezes quantum fluctuations in a laser light based on the ability to reduce quantum fluctuation for one of the two conjugate physical opinions according to the minimum uncertainty relation, at the sacrifice of increase in the other quantum fluctuation, and no attempt is made to apply this technique to the spectrum slices having a large intensity noise. As shown in FIG. 20, however, an optical-limiter effect of restraining an excess light intensity using the gain saturation of optical amplifies is effective in reducing intensity noise.
That is, a stabilized single-mode light source according to the present invention has an optical amplifying medium with gain saturation introduced into the optical ring to restrain laser oscillation. The gain saturation determines a fixed light output (a saturation output) independently of an input light intensity (a), as shown in FIG. 20. Thus, by appropriately optimizing the saturation output, the upper limit (msat) of the photon flow rate (m) can be reduced below a laser oscillation threshold mth for an individual population to hinder laser oscillation.
Consequently, if the above optical amplifier has gain saturation, intensity noise is substantially restrained because a light passes through the optical amplifier with gain saturation a large number of times according to the configuration of the present invention.
Furthermore, the light generation method according to the present invention includes optical filter control means for controlling the center transmission wavelength of the optical filter, the optical filter control means having a data-storage device for storing data of center transmission wavelengths versus control parameters of optical filters for determining the center transmission wavelength of the optical filter, the optical filter control means operative when a center transmission wavelength is provided as an instructive value, for reading from the data-storage device, the data of center transmission wavelengths versus control parameters of optical filters and controlling the optical filter so that the center transmission wavelength of the optical filter equals the center transmission wavelength provided as the instructive value.
With this configuration, if the optical filter is used for filtering and when a center transmission wavelength is provided as an instructive value, the optical-filter control means reads the data of center transmission wavelengths versus control parameters of optical filters from the data-storage device controls the optical filter based on the read data so that the center transmission wavelength of the optical filter be equal to the center transmission wavelength provided as the instructive value. As a result, a single-mode light which has a center transmission wavelength equal to the center transmission wavelength provided as the instruction value can be obtained.
Furthermore the light source according to the present invention uses a semiconductor optical amplifier as the optical amplifier having gain saturation.
The semiconductor optical amplifier, as used herein, is structured to have a double heterojunction that can realize an inverse distribution upon a current injection as in semiconductor lasers and to have an optical waveguide formed therein. The semiconductor optical amplifier is also structured to preclude end-surface reflection in order to prevent laser oscillation, so that a light is input from one end surface and output from the other end surface after being amplified while propagating through the optical waveguide. In such a semiconductor optical amplifier, the density of carriers contributing to the inverse distribution varies at a high speed depending on the input light intensity. As a result, such a semiconductor optical amplifier reacts even to components with fast variations in input light intensity and amplifies them.
Another feature of the semiconductor optical amplifier having the above characteristic is that due to a limit on the capacity with which injected electrons are stored as carriers contributing to the inverse distribution, a large optical input cannot be subjected to optical amplification based on a sufficient induced emission, resulting in a large gain saturation.
The use of the semiconductor optical amplifier having these characteristics enables light intensity noise to be substantially restrained. Furthermore, noise can be restrained in high frequency bands. Thus, a single-mode light with low noise can be obtained in a frequency band to which the field of the optical communications systems or the like is directed ( less than 40 GHz).
In addition, the light source according to the present invention controls polarization of a light input to the semiconductor optical amplifier.
Due to its structure similar to that of a semiconductor laser, the semiconductor optical amplifier has a minor polarization-dependent gain characteristic. For a linearly polarized light, such a semiconductor optical amplifier shows a gain characteristic dependent on a polarization direction. Thus, if the polarization direction of an input light does not align with that of the semiconductor optical amplifier, the nominal gain decreases. With a configuration using the above semiconductor optical amplifier, a light passing through the semiconductor optical amplifier is output as one similar to a linearly polarized light despite a depolarized state of the input light, whereby such polarization affects reducing the net gain.
The above configuration capable of controlling polarization, however, can compensate for the polarization dependency to allow the semiconductor optical amplifier to provide a high gain, thereby improving the effect of gain saturation and increasing outputs to provide a stable and high-output single-mode light.
Furthermore, the light source according to the present invention has another optical amplifier placed in a transmission section of the above optical filter in order to improve the light intensity, which is limited by the gain saturation of the semiconductor amplifier. The another optical amplifier may be comprised of a rare-earth-element-doped optical-fiber amplifier or a semiconductor optical amplifier.
Such a configuration enables a high-output single-mode light despite the use of a semiconductor optical amplifier that obtains a large gain saturation at the sacrifice of an absolute gain.
Furthermore, according to the light source of the present invention, the optical filter comprises a disc-shaped planar substrate and filters parallel lights passing through the disc perpendicularly or almost perpendicularly to its surface in such a manner that the center transmission wavelength is varied using as a variable a viewing-angle around a rotation symmetry axis of the disc. Means for determining the viewing-angle comprises a viewing-angle detection means consisting of detection of a mark written on the disc. Using a data-storage device for storing data on the center transmission wavelength using as variables the viewing-angle and a temperature measured near the optical filter, the viewing-angle of the optical filter is controlled so that the center transmission wavelength of the optical filter equals a center transmission wavelength provided as the instructive value. At the same time, the temperature measured near the optical filter is detected to constantly fine-tune the viewing-angle of the optical filter so that the center transmission wavelength of the optical filter equals the instructive value. The wavelength characteristic that the center transmission wavelength varies using the viewing-angle around the rotation symmetry axis of the disc is provided by a dielectric multilayer film band transmission optical filter having a cavity layer thickness proportional or almost proportional to the viewing-angle.
With such a configuration, by calibrating a center transmission wavelength of a disc-shaped dielectric-multilayer-film optical filter used as the optical filter, a single-mode light with a center transmission wavelength equal to the indicated wavelength can be obtained despite the simple configuration and without adverse effects of an environment temperature used for the light source.
Furthermore, the light source according to the present invention uses an ultrasonic motor as means for varying the viewing-angle of the disc-shaped optical filter.
The ultrasonic motor generates a transverse wave (a wave vibrating in a direction perpendicular to a propagating direction) on a surface to carry an object in contact with the surface on a wave front of the traveling wave based on frictional force. Such a motor is characterized by its small size, high drive force, and ability to hold an object at the same position by friction force.
This configuration does not only enable the above disc-shaped optical filter and its control system to be compactly assembled but also maintains optimal conditions under sequential control, by reading from the data-storage device, transmission wavelength data comprising the viewing-angle of the disc-shaped optical filter stored in the data-storage device so that the center transmission wavelength of the optical filter equals a center transmission wavelength provided as the instructive value, and by setting the center transmission wavelength at an optimal value, although temperature varies. In addition, if the temperature varies, then it is monitored and based on the read transmission wavelength data comprising the viewing-angle of the disc-shaped optical filter, the viewing-angle of the optical filter can be corrected to obtain a center transmission wavelength equal to the instructive value. This configuration stably provides a single-mode light having a center transmission wavelength equal to the indicated wavelength.
Furthermore, according to the light source of the present invention, the optical filter comprises an acoustooptical filter for controlling the center transmission wavelength depending on the frequency of an electrical oscillator; the light source has a data-storage device for storing a center transmission wavelength obtained using the frequency as a variable, as data of center transmission wavelengths versus control parameters of optical filters; and when a center transmission wavelength is provided as an instructive value, the data is read from the data-storage device and the frequency of the electrical oscillator for controlling the optical filter is controlled so that the center transmission wavelength of the optical filter equals the instructive value.
This configuration enables the center transmission wavelength of the optical filter to be switched at a high speed within a range of speeds at which the frequency of the electrical oscillator is controlled, thereby allowing the center transmission wavelength of a single-mode light to be promptly set at this speed depending on a timing with which the instructive value is received.
In addition, according to a stabilized single-mode light source, one or more optical amplifying media, an optical filter, an optical power divider, and an optical attenuator is connected together in the form of a ring to form an optical ring; at least one of the optical amplifying media has gain saturation; and attenuation provided by the optical attenuator is adjusted so that a mode circulating through the optical ring is kept equal to or smaller than a laser oscillation threshold, so that a monochromatic light of a wavelength selected by the optical filter is branched and output from the optical power divider.
That is, a light output from the optical amplifying medium is spectrum-sliced by the optical filter, then the optical ring is formed in which an input is led to the optical gain medium via the optical attenuator, and finally the excitation level of the optical gain medium and attenuation provided by the optical attenuator are adjusted to allow the light to circulate through the ring a number of times while being attenuated.
When the optical filter is inserted into the optical ring including the optical amplifying medium, bands in which spontaneous emission occurs are limited to within the band of the optical filter. Thus, all populations are subjected to photon counting even during a short observation time. Consequently, the average of the average photon flow rates of the populations is fixed regardless of the observation time. The probability density function W(xcexd) approaches a delta function, and even for a spectrum slice circulating through the optical ring while being attenuated, the probability density function PT(m) for the photon counting statistics shows the poisson distribution, as shown in FIG. 7B.
If the gain of the optical amplifying medium is not saturated, the probability density function converges on zero but does not reach exact zero despite an infinite value of the photon flow rate (m). Thus, there is a probability that a photon flow rate equal to or larger than an oscillation threshold of the optical ring occurs and that the rate meets a wavelength required for resonance with the optical ring as well as polarization conditions to lead to laser oscillation, as shown in FIG. 7A. Since in a laser oscillation state, the average photon flow rate is fixed independently of the population, the photon counting statistics for all populations shows the poisson distribution, as shown in FIG. 7B. When an oscillation mode grows from a population, the extension of the probability density function for the photon counting statistics for all populations is smaller than that for a noise wave.
Such a laser oscillation state, however, is difficult to sustain for a long time due to fluctuations in the optical ring (for example, fluctuations in fiber length). Thus, the oscillation state rapidly changes to a non-oscillation state to cause a large intensity noise. In the non-oscillation state, the individual populations compete again, and one dominant population enters the laser oscillation state to generate a similar intensity noise. Such intensity noise caused by laser oscillation substantially obstructs optical communication systems.
A light circulating through the optical ring while being attenuated has its band with reduced due to passage through the optical filter a number of times. When the filter transmission function is defined as T(xcex) and a net loss per circulation is defined as (xcex3), a full transmission function Teff(xcex) is expressed as follows:
Teff(xcex)=T+xcex3T2+xcex32T3+ . . . =T/(1xe2x88x92xcex3T)xe2x80x83xe2x80x83(11)
If T is a Lorentz transmission function and the full width at half maximum is 0.1 nm, the spectrum width of an output light is 0.01 nm at xcex3=xe2x88x920.05 dB. Even for such a spectrum slice of a reduced width, the photon counting statistics follows the poisson distribution.
In addition, the light generation method according to the present invention outputs a single-mode light by filtering a spontaneous emission using an optical filter, and comprises using an optical amplifier as a light source for generating the spontaneous emission, inputting to the optical amplifier a spontaneous emission having a larger bandwidth than the transmission bandwidth of the optical filter and including the center transmission wavelength of the optical filter in this band in order to increase, in the optical amplifier, the probability density of light emission of the single-mode light near the center transmission wavelength, and using the optical filter to filter the spontaneous emission amplified by the optical amplifier. The xe2x80x9csingle-mode lightxe2x80x9d refers to a light showing a unimodal shape in the wavelength domain (a light having significantly high wavelength components only in a particular band).
Furthermore, the light generation method according to the present invention outputs a single-mode light having wavelength components in a particular band of the spontaneous emission band by obtaining the single-mode light from a spontaneous emission having wavelength components over a wide band in a wavelength domain, and the single-mode light is obtained by carrying out, at least once, the process of filtering the spontaneous emission using an optical filter having at least the transmission bandwidth of the particular band, using an optical amplifier to amplify a light transmitted through the optical filter, and filtering the light using the optical filter having at least the transmission bandwidth the particular band.
According to this method, the spontaneous emission is first filtered by the optical filter. The light transmitted through the optical filter has the wavelength components in the particular band because the other components in the spontaneous emission band are filtered. Then, the light transmitted through the optical filter is amplified by the optical amplifier and filtered by the optical filter. After this amplification, the light transmitted through the optical filter contains the spontaneous emission amplified by the optical amplifier, but this spontaneous emission is filtered by the subsequent optical filter, resulting in the wavelength components in the particular band being particularly amplified. The single-mode light is obtained by carrying out a process comprising such amplification and filtering at least once.
The process of using the optical amplifier to amplify the light transmitted through the optical filter and filtering the light using the optical filter having at least the transmission bandwidth of the particular band may be carried out at least once but may be executed a number of times as required.
In addition, the optical filter for filtering the spontaneous emission may be identical to or different from the optical filter for filtering the light amplified by the amplifier.
Furthermore, the light generation method according to the present invention comprises the steps of filtering the spontaneous emission using a first optical filter having at least the transmission bandwidth of the particular band, using the optical amplifier to amplify a light transmitted through the first optical filter, and filtering the amplified light from the optical amplifier using a second optical filter having at least the transmission bandwidth the particular band, in order to obtain a light transmitted through the second optical filter as the single-mode light.
The first and second optical filters may be configured to have identical characteristics or to have different characteristics if they have at least the transmission bandwidth of the particular band. Since the profile of the single-mode light is determined by a transmission profile of the second optical filter, the center transmission wavelengths of these optical filters need not be exactly equal. For example, the first optical filter may have the transmission bandwidth of a wider band including the particular band, while the second optical filter may have the transmission bandwidth only of the particular band.
Furthermore, according to the light generation method of the present invention, the second optical filter has a center transmission wavelength identical to the center transmission wavelength of the transmission band of the first optical filter and has a transmission bandwidth smaller than or identical to the transmission bandwidth of the first optical filter.
In addition, the light generation method according to the present invention comprises the steps of filtering the spontaneous emission using an optical filter having at least the transmission bandwidth of the particular band, using the optical amplifier to amplify a light transmitted through the optical filter and feeding the light back to the optical filter to obtain a light transmitted through the optical filter as the single-mode light.
Furthermore, according to the light generation method of the present invention, the filtering is carried out when a center transmission wavelength is provided as an instructive value, by reading data of center transmission wavelengths versus control parameters of optical filters, from a data-storage device with this data stored therein and controlling the optical filter based on the read data so that the center transmission wavelength of the transmission band of the optical filter equals the center transmission wavelength provided as the instructive value.
In addition, according to the light generation method of the present invention, the optical filter is a disc-shaped optical filter having a predetermined transmission bandwidth and a circularly changed central transmission wavelength, and carries out filtering by changing the center transmission wavelength depending on a rotation angle of the disc-shaped filter in such a manner that light is incident on a surface of the disc-shaped filter at a fixed position thereof to pass through in a rotation axis direction, wherein the filtering is carried out when a center transmission wavelength is provided as an instructive value, by reading data of center transmission wavelengths versus control parameters of optical filters, from a data-storage device with this data stored therein, the data comprising different center transmission wavelengths of the optical filter associated with corresponding rotation speeds of the disc-shaped filter, and controlling the viewing-angle of the disc-shaped filter of the optical filter based on the read data so that the center transmission wavelength of the transmission band of the optical filter equals the center transmission wavelength provided as the instructive value.
On the other hand, the light source of the present invention outputs a single-mode light by filtering a spontaneous emission using an optical filter, and includes an optical amplifier for generating the spontaneous emission. The light source inputs to the optical amplifier a spontaneous emission having a bandwidth larger than the transmission bandwidth of the optical filter and including the center transmission wavelength of the optical filter in this band in order to increase, in the optical amplifier, the probability density of light emission of the single-mode light near the center transmission wavelength, and uses the optical filter to filter the spontaneous emission amplified by the optical amplifier.
With this configuration, the spontaneous emission having a bandwidth larger than the transmission bandwidth of the optical filter and including the center transmission wavelength of the optical filter in this band is input to the optical amplifier, thereby increasing, in the optical amplifier, the probability density of light emission of the single-mode light near the center transmission wavelength. Then, the spontaneous emission amplified by the optical amplifier is filtered by the optical filter.
Furthermore, the light source of the present invention outputs a single-mode light having wavelength components in a particular band of a spontaneous emission band by obtaining the single-mode light from a spontaneous emission having wavelength components over a wide band in a wavelength domain. In this case, the single-mode light is obtained by carrying out, at least once, the process of filtering the spontaneous emission using an optical filter having at least the transmission bandwidth of the particular band, using an optical amplifier to amplify a light transmitted through the optical filter, and filtering the light using the optical filter having at least the transmission bandwidth of the particular band.
With this configuration, the spontaneous emission is first filtered by the optical filter. Thus, the light transmitted through the optical filter has the wavelengths in the particular band because the other wavelength components are filtered. Then, the light transmitted through the optical filter is amplified by the optical amplifier and filtered by the optical filter. After this amplification, the light transmitted through the optical filter contains the spontaneous emission amplified by the optical amplifier, but this spontaneous emission is filtered by the subsequent optical filter, resulting in the wavelength components in the particular band being particularly amplified. The single-mode light is obtained by carrying out a process comprising such amplification and filtering at least once.
The process of using the optical amplifier to amplify the light transmitted through the optical filter and filtering the light using the optical filter having at least the transmission bandwidth of the particular band may be carried out at least once but may be executed a number of times as required.
In addition, the optical filter for filtering the spontaneous emission may be identical to or different from the optical filter for filtering the light amplified by the amplifier.
Furthermore, the light source according to the present invention comprises a first optical filter having at least the transmission bandwidth of the particular band and a second optical filter having at least the transmission bandwidth of the particular band. The light. source filters the spontaneous emission using the first optical filter, uses the optical amplifier to amplify a light transmitted through the first optical filter, and filters the amplified light from the optical amplifier using the second optical filter to obtain a light transmitted through the second optical filter as the single-mode light.
With this configuration, the spontaneous emission is filtered by the first optical filter, and the light transmitted through the first optical filter is amplified by the optical amplifier. Then the amplified light from the optical amplifier is filtered by the second optical filter to obtain the light transmitted through the second optical filter as the single-mode light.
The first and second optical filters may be configured to have identical characteristics or to have different characteristics if they have at least the transmission bandwidth of the particular band. Since the profile of the single-mode light is determined by a transmission profile of the second optical filter, the center transmission wavelengths of these optical filters need not be exactly equal. For example, the first optical filter may have the transmission bandwidth of a wider band including the particular band, while the second optical filter may have the transmission bandwidth only of the particular band. However, in order to ensure the wavelength accuracy of the single-mode light, the center transmission wavelength of the second optical filter must be precisely calibrated and temperature and atmospheric pressure must be compensated for.
Furthermore, according to the light source of the present invention, the second optical filter has a center transmission wavelength identical to the center transmission wavelength of the transmission band of the first optical filter and has a transmission bandwidth smaller than or identical to the transmission bandwidth of the first optical filter.
In addition, the light source according to the present invention comprises an optical filter and an optical filter having at least the transmission bandwidth of the particular band. The light source filters the spontaneous emission using the optical filter, uses the optical amplifier to amplify a light transmitted through the optical filter, and feeds the light back to the optical filter to obtain a light transmitted through the optical filter as the single-mode light.
With this configuration, the spontaneous emission is filtered by the optical filter, the light transmitted through the first optical filter is amplified by the optical amplifier, and. Then, the amplified light from the optical amplifier is filtered by the second optical filter to obtain the light transmitted through the second optical filter as the single-mode light.
Furthermore, the light source according to the present invention comprises optical filter control means for controlling the center transmission wavelength of the optical filter, and the optical filter control means has a data-storage device storing data of center transmission wavelengths versus control parameters of optical filters for determining the center transmission wavelength of the optical filter. When a center transmission wavelength is provided as an instructive value, the optical filter control means reads from the data-storage device the data of center transmission wavelengths versus control parameters of optical filters and controls the optical filter based on the read data so that the center transmission wavelength of the optical filter equals the center transmission wavelength provided as the instructive value.
With this configuration, if the optical filter is used for filtering and when a center transmission wavelength is provided as an instructive value, the optical-filter control means reads from the data-storage device the data of center transmission wavelengths versus control parameters of optical filters and controls the optical filter based on the read data so that the center transmission wavelength of the optical filter equals the center transmission wavelength provided as the instructive value. As a result, the light incident on the optical filter is filtered for the transmission bandwidth of the center transmission wavelength provided as the instruction value.
If a plurality of different optical filters (a first and a second optical filters) are used, these filters are preferably controlled so that their center transmission wavelengths are simultaneously changed.
Furthermore, according to the light source of the present invention, the optical filter is a disc-shaped optical filter having a predetermined transmission bandwidth and a circularly changed central transmission wavelength, and carries out filtering by varying the center transmission wavelength depending on a viewing-angle of the disc-shaped filter in such a manner that light is incident on a surface of the disc-shaped filter at a fixed position thereof to pass through a rotation axis direction. The light source comprises optical filter control means for controlling the center transmission wavelength of the optical filter, and the optical filter control means has a data-storage device for storing data of center transmission wavelengths versus control parameters of optical filters, the data comprising different center transmission wavelengths of the optical filter associated with corresponding viewing-angles of the disc-shaped filter. When a center transmission wavelength is provided as an instructive value, the optical filter control means reads from the data-storage device the data of center transmission wavelengths versus control parameters of optical filters, and controls the viewing-angle of the disc-shaped filter of the optical filter based on the read data so that the center transmission wavelength of the transmission band of the optical filter equals the center transmission wavelength provided as the instructive value.
With this configuration, if the optical filter is used for filtering and when a center transmission wavelength is provided as an instructive value, the optical-filter control means reads from the data-storage device the data of center transmission wavelengths versus control parameters of optical filters and controls the optical filter based on the read data so that the center transmission wavelength of the optical filter equals the center transmission wavelength provided as the instructive value. As a result, the light incident on the optical filter is filtered for the transmission bandwidth of the center transmission wavelength provided as the instruction value.
If a plurality of different optical filters (a first and a second optical filters) are used, these filters are preferably controlled so that their center transmission wavelengths are simultaneously changed.
The above and other objects, effects, features, and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.